it detect the user-agent , and for those who said selenium..it is detectable for some web applications like twitter , for example I have create a bot for register new user in twitter with browser module which is based on selenuim .. and I got blocked from twitter ...and I guess twitter knows based on the time spend to fill the registration form which is second or two...so I did sleep 2,3 or 4 second every time filling a field ..so it's working ......and for google use list of user-agents and time sleep. It's working for me.